Re: [linux-usb-devel] Re: [PATCH 00/10] usb-serial: Switches from spin lock to atomic_t.

[Oliver Neukum]

Am Mittwoch, 7. Dezember 2005 16:40 schrieben Sie:
> On Wed, 2005-12-07 at 16:37 +0100, Oliver Neukum wrote:
> > Am Mittwoch, 7. Dezember 2005 16:22 schrieb Arjan van de Ven:
> > > > On the other hand, Oliver needs to be careful about claiming too much.  In 
> > > > general atomic_t operations _are_ superior to the spinlock approach.
> > > 
> > > No they're not. Both are just about equally expensive cpu wise,
> > > sometimes the atomic_t ones are a bit more expensive (like on parisc
> > > architecture). But on x86 in either case it's a locked cycle, which is
> > > just expensive no matter which side you flip the coin...
> > 
> > You are refering to SMP, aren't you?
> 
> yes.
> on UP neither is a locked instruction ;)

But the atomic variant has to guard against interrupts, at least on
architectures that do load/store only, hasn't it? AFAICT it is even
theoretically impossible to tell for the compiler whether a function
is always called with interrupts off.

	Regards
		Oliver
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